The invention disclosed and taught herein generally relates to self-contained actuator systems and, in particular, to a self-contained energy efficient hydraulic actuator system.
An actuator is the mechanism by which a control system acts upon an environment. It is operated by a source of energy typically an electric current, a motor, and hydraulic fluid pressure or pneumatic pressure, which converts that energy into motion.
A hydraulic actuator typically consists of a cylinder that uses hydraulic power to facilitate mechanical operation. The mechanical motion gives an output in terms of linear, rotary or oscillatory motion. The hydraulic cylinder consists of a hollow cylindrical tube along which a piston can slide. The term double acting is used when pressure is applied on each side of the piston. A difference in pressure between the two sides of the piston results in motion of piston to either side. The term single acting is used when the fluid pressure is applied to just one side of the piston. If the piston moves in only one direction, a spring being frequently used to give the piston a return stroke.
Conventionally, a hydraulic linear actuator is connected to a remote supply of pressurized hydraulic fluid through a closed network of pipes and control valves. However, there are applications where it is desirable for a hydraulic linear actuator to be freestanding and mobile, having a prime mover, hydraulic pump, and a closed hydraulic fluid control system all integrated with and located proximate to the linear actuator. Such compact freestanding actuators are particularly suitable for industrial valve applications and remote locations where such valves may be located.
Prior art freestanding hydraulic actuators are disclosed in U.S. Pat. Nos. 2,640,323 and 2,640,426 to McLeod; U.S. Pat. No. 5,144,801 to Scanderbeg et al.; U.S. Pat. No. 8,336,613 to Ramsey et al.; and U.S. Pat. No. 6,892,534 to Silva et al.
Self-contained hydraulic actuator systems having closed hydraulic systems can incorporate a servo valve. The servo valve alters the direction of the fluid in the system, and thus controls the movement of a double acting hydraulic cylinder. One of the drawbacks of using the servo valve to change the fluid direction is the servo valves' continuous internal leakage that requires a continuous supply of hydraulic fluid from the prime mover driven pump. Hydraulic servo valves may also fail because of particulate contamination that can be carried into the tight clearances between the moving components of a servo valve.
Self-contained hydraulic actuator systems can also incorporate hydraulic pumps (e.g., a bi-directional hydraulic pump). These systems require bi-directional motors to drive the hydraulic pump. The movement of a double acting hydraulic cylinder is controlled by the speed and direction of the hydraulic pump as fluids flow through the system.
Published U.S. Patent Application No. 2007/0101711 to Debus discloses the use of an AC induction motor driven by a variable frequency drive (VFD), and the hydraulic pump speed and directions are controlled by the motor. However, a VFD driven motor has limited torque available at low RPM, may not start under load, and resists rapid RPM and direction changes. Even though the Debus application discloses the use of a bypass leak path which allows the motor to run at some minimum RPM when the actuator is stationary, the motor is required to run continuously even under the hold position because there is no separate provision for load locking. The continuous running of the motor results in expenditure of unnecessary energy, shortened life of the motor, frequent repairs and ultimately extra cost.
U.S. Pat. No. 7,640,736, to Arbel, describes a hydraulic linear actuator system including a pump that is configured to rotate in a single direction at a substantially constant velocity. Arbel uses a single direction motor, and a bi-directional stepper motor to change the direction of the pump and flow of fluid. Both the direction and flow rate of fluid is controlled by adjusting the positional relationship between the stator and the rotor of the pump. However, Arbel fails to provide for load locking. Another drawback of Arbel is that both the pump and prime mover must run in order to maintain a static actuator position.
Scanderbeg et al., U.S. Pat. No. 5,144,801, discloses a freestanding electro-hydraulic actuator having an electric motor disposed in a hydraulic fluid reservoir and connected to drive a hydraulic fluid pump. Scanderberg discloses that the electric motor drives the hydraulic pump “on demand.” On demand basis is associated with motor speed changes “generating only the required pressure and flow,” but does not cycle on and off. When the actuator achieves desired position, the motor slows down, but continues to run slowly to maintain position. Scanderberg fails to provide a separate provision for load locking.
Duff, U.S. Pat. RE39,158, discloses a hydraulic system manifold having a body, a counterbalance in the body and a flow controller in the body. Duff's patent is directed to an actuator that provides load locking without use of the motor through the use of a manifold having counterbalance and mechanical flow locking valves. Duff's flow locking valves operate on system or line pressure and hold the actuator in place when the motor is off. However the pilot operated check valves introduce positioning resolution issues. Glomeau, U.S. Pat. No. 4,766,728, overcomes the pilot operated check valve positioning resolution issues with the disclosure of the Flow Matching Valve.
There are a number of drawbacks associated with the disclosures in the prior art. One major drawback is that the actuator holding ability is wholly dependent on continuous operation of the prime mover or mechanical control valves to lock fluid in the hydraulic cylinder. Conventional freestanding hydraulic linear actuators do not normally have the necessary motor, pump, and/or valve configuration to accomplish the task of load locking, and thus depend on the prime mover to maintain fluid pressure for load locking. This increases power consumption and reduces component life as the prime mover and pump are needed to operate continuously.
Another major drawback is that prime mover and pump operate frequently and in some applications, continuously to make up for the servo valve leakage rate. This greatly limits actuator frequency response and positioning accuracy and repeatability.
Thus, there is a need in the art to provide an energy efficient actuator system that can be operated in any orientation, which provides for load locking, without the need of a servo valve, and where the system has the capability to stop the motor at the stationary state of the piston, and to restart the motor from zero RPM under load.